(CNN) -- Sen. John McCain, the presumptive Republican presidential nominee, is pushing hard to attract independent voters, arguing that it's the best way to beat Sen. Barack Obama in the fall.

Sen. John McCain's speech last week was criticized within GOP circles for its poor visuals.

A CNN/Opinion Research Corp. poll released Friday shows that the independent vote is up for grabs. The candidates split independents, each receiving the support of 45 percent of those voters surveyed. The poll's margin of error was plus or minus 4.5 percentage points. It was conducted Wednesday through Thursday.

Outlining strategy on its Web site, the McCain camp said it can win over independents by challenging the presumptive Democratic nominee on issues ranging from taxes to the environment.

On Friday, McCain attacked Obama's record on the environment during a campaign stop in the Florida Everglades.

"Sen. Obama has no record of being involved in this issue that I know of," he said. "I will stick by my record and my commitment of many years to reduce greenhouse gas emissions."

On Tuesday, McCain denounced Obama's position on taxes and trade during the National Small Business Summit in Washington.

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"No matter which of us wins in November, there will be change in Washington," McCain said. "The question is, what kind of change? Will we enact the single largest tax increase since the Second World War, as my opponent proposes, or will we keep taxes low for families and employers?

"This election offers Americans a very distinct choice about what kind of change we will have. This is especially true for the small business community."

Obama has blasted McCain for what he says is a continuation of President Bush's economic policies.

On Tuesday, Obama also showed his readiness to challenge McCain on an issue crucial to independent voters: rising health care costs.

"John McCain doesn't have a comprehensive plan to control rising health care costs. Instead, the centerpiece of his plan is eliminating tax breaks for employers that provide health care," Obama said in St. Louis, Missouri. "The group he's addressing today did a survey to see what their members thought of this idea, and more than 70 percent oppose his proposal."

McCain's campaign manager, Rick Davis, admitted that the political environment is "the worst in modern history for Republicans" during a recent message on the campaign Web site.

But McCain advisers said they also will target "disaffected Democrats" who may have backed Sen. Hillary Clinton during the primary fight and are disappointed she didn't win the nomination. Of particular importance could be Democrats in swing states such as Ohio and Pennsylvania who told pollsters they would not vote for Obama.

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The CNN/Opinion Research poll found that 17 percent of those surveyed who had backed Clinton were considering voting for McCain. The margin of error on that question was plus or minus 7.5 percentage points.

"He is a Republican," Democratic pollster Peter Hart said. "He happens to believe we stay in Iraq as long as we can, and he has voted with George Bush 89 percent of the time."

McCain also has stirred grumbles within his party. For months, Republican operatives privately have expressed concern about how the McCain campaign is executing its strategy.

GOP worries spilled into the open last week when McCain's prime-time speech was criticized for its poor visuals and negative message, compared with Obama's address before a raucous crowd in St. Paul, Minnesota, when he proclaimed that he had secured the nomination.

A green backdrop for McCain was especially targeted for derision.

"Almost every Republican I've talked to is alarmed that the McCain campaign doesn't seem up to the task of electing John McCain," conservative New York Times columnist William Kristol later wrote.

McCain advisers said they're aware that Republicans are worried about the fight against Obama, but a McCain spokeswoman said they had risen above pundits' doubts before and would do so again.